1. Field of the Invention
This invention relates to a technique relating to holding, transfer and the like of a substrate by suction, which is used, for example, for manufacturing semiconductor devices.
2. Description of the Related Art
FIG. 6 illustrates a conventional apparatus for holding a semiconductor wafer by a vacuum suction method, which is used, for example, for a semiconductor-device manufacturing apparatus. In this substrate holding apparatus, conveying chuck 131 for holding wafer 141 by suction is mounted on a conveying mechanism having an X-stage 133 moving in the x-direction (the lateral direction in FIG. 6) and a Z-stage 134 moving in the z-direction (a direction perpendicular to the plane of FIG. 6). Wafer 141 is transferred between conveying chuck 131, and wafer chuck 132 for holding wafer 141 by suction during an exposure operation. These components are housed within chamber 142 maintained under a reduced pressure.
Conveying chuck 131 is connected to distributor 139 by connection line 135 having first valve 137 at a midpoint thereof. Wafer chuck 132 is connected to distributor 139 by connection line 136 having second valve 138 at a midpoint thereof. Distributor 139 is connected to pump 140.
Each of conveying chuck 131 and wafer chuck 132 holds and releases wafer 141 by a suction operation of pump 140. Each of valves 137 and 138 is a three-way valve, which can switch between a first state in which each of conveying chuck 131 and wafer chuck 132 communicates with pump 140, respectively, and a second state in which each of conveying chuck 131 and wafer chuck 132 communicates with the atmosphere within chamber 142, respectively.
An explanation will now be provided of a case in which wafer 141 is transferred from conveying chuck 131 to wafer chuck 132. First, in order to hold wafer 141 by conveying chuck 131 by suction, first valve 137 is maintained in the first state in which conveying chuck 131 communicates with pump 140, and pump 140 is operated to hold wafer 141 by suction by conveying chuck 131. At that time, second valve 138 is maintained in the second state in which wafer chuck 132 communicates with the atmosphere of chamber 142, so that the pressure within second vacuum line 136 equals the pressure of the atmosphere within chamber 142.
Thereafter, by driving X-stage 133, wafer 141 held by conveying chuck 131 by suction is moved onto the surface of wafer chuck 132. Then, by driving Z-stage 134, conveying chuck 131 is moved until wafer 141 contacts the surface of wafer chuck 132. When wafer 141 has contacted the suction surface of wafer chuck 132, second valve 138 is switched to the first state, whereby wafer 141 is held by suction by wafer chuck 132. That is, wafer 141 is held by suction by both conveying chuck 131 and wafer chuck 132. Thereafter, first valve 137 is switched to the second state in which conveying chuck 131 communicates with the atmosphere of chamber 142. Thus, the holding force of conveying chuck 131 by suction disappears, and wafer 141 is held by suction only by wafer chuck 132. Thereafter, by sequentially driving Z-stage 134 and X-stage 133, conveying chuck 131 is returned to the original position. Thus, the tranfer of wafer 141 to wafer chuck 132 is completed.
While wafer 141 is held by wafer chuck 132, a circuit pattern is exposed and transferred onto wafer 141 by an exposure apparatus (not shown). Thereafter, by driving X-stage 133 and Z-stage 134, conveying chuck 131 is moved to the position of wafer chuck 132 in order to transfer wafer 141. This tranfer operation is inverse to the above-described transfer operation of wafer 141 from conveying chuck 131 to wafer chuck 132.
The above-described conventional approach, however, has the following problems to be solved.
(1) The minimum differential pressure necessary for holding the substrate by suction, i.e., the critical differential pressure dPL depends on the following items:
1) The surface roughness of the holding surface of the substrate holding apparatus.
2) The flatness of the holding surface of the substrate holding apparatus.
3) The suction area of the holding surface of the substrate holding apparatus.
When the substrate is held, transferred and conveyed by setting the suction pressure to the critical differential pressure dPL, the subtrate cannot be held and leaves the surface of the chuck if, for example, a part of the coated resist moves to the back of the substrate, or dust adheres to the suction surface. On the other hand, if the suction pressure is set to the maximum differential pressure, time is needed until the suction pressure reaches the set pressure, causing a decrease in the throughput of the operation.
(2) If the substrate is conveyed at a high speed, the conveying speed and the acceleration until the speed reaches the conveying speed increase. Hence, it is necessary to prevent the substrate from dropping by increasing the differential pressure between the suction pressure of the conveying chuck and the pressure of the atmosphere of the chamber. On the other hand, when performing exposure using X-rays, it is necessary to transfer heat generated at a mask to a temperature-controlled wafer chuck. For that purpose, He (helium) gas is usually supplied between the mask and the wafer, and the wafer and the wafer chuck to increase the efficiency of heat conduction. If the differential pressure for the conveying chuck is increased as described above, the differential pressure for the wafer chuck is also increased. As a result, the degree of vacuum between the wafer and the wafer chuck increases, causing a reduction in the amount of He gas. The thermal contact resistance between the wafer and the wafer chuck thereby increases, causing a decrease in the efficiency of heat conduction.
(3) When a plurality of chucks, such as the above-described conveying chuck, wafer chuck and the like, are provided, since the suction area, the surface roughness and the like of the substrate holding surface differ for each chuck, the suction force differs even if the same suction pressure is provided.